I've used my Uber Bivy three times now but have had no luck getting it to rain while in it. I'm 6'4" and 240Lbs. Plenty of room for me and my gear inside the bivy. It's as if the bivy isn't there as there is no feeling of constraint.
Last Friday night I used it at Rancheria Falls in Yosemite expecting rain but got none. It didn't get cold enough to freeze water bottles but I was inside the bivy without any head covering, no mittens, and a light top shirt. Very warm and cozy with a 25 degree bag. I would say the bivy gives at least 15-20 degrees of extra warmth. I tested it with the mesh open to the outside and with the mesh zippered. An amazing difference in the warmth with the mesh closed.
If you are a bivy person try this one, it's a keeper.
